1. Field of the Invention
The present invention relates to a heat transfer printer which melts a hot-melt ink of a transfer material by a thermal head and which transfers the ink onto a recording paper sheet for printing.
2. Description of the Prior Art
FIG. 1 shows the construction of a heat transfer printer. A thermal head 4 as a recording head of 7.times.1 heat-generating dot matrix construction (in the longitudinal and transverse directions, respectively) is urged by a spring 5 against a platen 1 of soft rubber through a printing paper sheet 2 as a recording medium and through a transfer ink ribbon 3 as an ink-bearing member which is prepared by applying to a film a hot-melt ink with a binder. The heat transfer ribbon 3 is fed from a supply reel 3A mounted on a carriage 6 as part of a head displacing mechanism to a take-up reel 3B mounted on the carriage 6, passing between the thermal head 4 and the paper sheet 2.
A spring 9 is mounted between the carriage 6 and one end of a lever 8 which is pivotable about a shaft 7 as a fulcrum on the carriage 6. The spring 9 biases the lever 8 clockwise about the shaft 7. A pulse motor 10 as a power source for transferring the ribbon is mounted to the other end of the lever 8. A ratchet wheel 11 is securely fixed on a motor shaft of this pulse motor 10. The ratchet wheel 11 and the transfer ink ribbon wound around the take-up reel 3B engage each other by the tension exerted by the spring 9 mounted to the end of the lever 8, so that they may be constantly urged against each other independently of the diameter of the coil of the transfer ink ribbon.
The carriage 6 is mounted to stationary shafts 12 and 13 which are parallel to the platen 1, so that the thermal head 4 may be displaced parallel to the platen 1. One end of a tooth belt 14 is fixed to one side of the carriage 6, and the other end of the belt 14 is fixed to the other side of the carriage 6 through pulleys 15 and 16. A gear 17 is formed integrally with the pulley 15 to engage with a gear 19 fixed on the shaft of a pulse motor 18 for the carriage, so that, upon rotation of the pulse motor 18, the tooth belt 14 may be driven to displace the carriage 6 parallel to the platen 1. The construction is such that a speed v of displacement of the carriage 6 is the same as a speed of displacement in the opposite direction of the ink ribbon driven by the pulse motor 10 for the ribbon, and corresponding parts of the transfer ink ribbon 3 and the printing paper sheet 2 are brought into contact with each other when the carriage 6 moves.
An initial switch 20 is turned on and off according to the movement of the carriage 6, for checking the position of the thermal head 4.
The heat transfer printer of the construction as described above is conventionally operated according to the timings of the chart shown in FIG. 2. When the carriage 6 is at the left end under the waiting condition, the initial switch 20 is kept on. When the same pulse is applied to the pulse motor 18 for the carriage and the pulse motor 10 for the ribbon as shown in FIG. 2, the carriage 6 is displaced to the right to turn off the initial switch 20, so that printing may be initiated beginning with the timing of the next pulse.
Since the thermal head 4 on the carriage 6 moves to the right at the speed v, and the transfer ink ribbon 3 is carried at the speed v over the carriage 6 in the opposite direction, the printing paper sheet 2 and the transfer ink ribbon 3 do not move relative to each other and are kept stationary at the part against which the thermal head 4 is urged. At the timing of a pulse t1 which is applied after the pulse at which the initial switch 20 is turned off, dots C2 and C7 of dots C1 to C7 of the thermal head 4 arranged in the column direction are energized for a predetermined period of time by a known technique to melt the hot-melt ink of the transfer ink ribbon 3 and to transfer the molten ink onto the printing paper sheet as dots of predetermined size of the first column of a character as shown in FIG. 3. At the timing of a next pulse t2, dots C1, C6 and C7 are heated for printing the next column of the character. In this manner, the thermal head 4 is displaced parallel to the platen 1 while the respective dots of the thermal head 4 are heated for each column of the character to print the desired character through the transfer ink ribbon 3.
However, with this method, the speed v of the carriage 6 becomes v.+-..DELTA.v18 in practice due to speed fluctuations of the motor which are, in turn, caused by load fluctuations. Meanwhile, the speed -v of the transfer ink ribbon 3 has small fluctuations and may also be represented as -v.+-..DELTA.v10 in practice due to errors in manufacture of the parts involved, the degree of engagement of the pawl of the ratchet wheel 11 with the tape, or the like. Therefore, the transfer ink ribbon 3 and the printing paper sheet 2 have a relative speed, although very small, which may be given by: EQU (v.+-..DELTA.v18)+(v.+-..DELTA.v10)=.+-..DELTA.v18.+-..DELTA.v10
In the heat transfer printer, the thermal head is, in general, urged against the platen with the transfer ink ribbon and the printing paper sheet interposed therebetween. Upon application of heat, the hot-melt ink in the transfer ink ribbon is transferred to the printing paper sheet. However, all the ink corresponding to the heated dots may not be transferred, and some may be left on the transfer ink ribbon between the printing paper sheet and the transfer ink ribbon having a speed relative to each other.
If there is a small error between the speed of the transfer ink ribbon and that of the printing paper sheet at timings of pulses t1 and t4 shown in FIG. 2, the hot-melt ink which is to be transferred from the transfer ink ribbon 3 to the printing paper sheet 2 is not completely transferred due to oscillations, but is partly returned to the transfer ink ribbon 3. This results in unclear printing as shown in FIG. 3.
At the timings of pulses t2, t3 and t5, the relative speed of the transfer ink ribbon and the printing paper sheet is zero. Therefore, the printing paper sheet and the transfer ink ribbon stick to each other. Then, when the transfer ink ribbon 3 is separated from the printing paper sheet to be taken up on the take-up reel 3B, the ink is transferred to the side of the printing paper sheet due to the difference in adhesiveness between the transfer ink ribbon 3 and the printing paper sheet. Accordingly, the dots are printed at a high density. In this manner, the printing is performed with irregular density with the conventional heat transfer printer.